On Tue, 10 Oct 2000, Rick Hulsey wrote:
It looks like the configuration that was used in the benchmark was a single volume that had 2 RAID groups (RAID group 0, and RAID group 1, presumably), and each of these RAID groups was cabled up to separate interfaces. I'm guessing that this configuration would yield a filesystem that is striped across both FC-AL interfaces. Since the RAID calculations are done at the RAID group level rather than at the volume level it makes sense that more than one RAID calculation can be done in parallel.
This triggered a question in my mind... <flipping through my 202 class notes...> Here we go. <ahem>
"A reading from the book of NetApp, Chapter "Performance Tuning," Page 7, Paragraph I of the April 2000 notes..." [0]
Hitz sayeth (I assume):
"WAFL writes data in a round-robin fashion to the RAID groups in a volume. Data is written to one RAID group at a time, one per consistency point. Because of the write allocation policy the filer will attempt to distribute writes evenly between RAID groups regardless of size."
My questions then are:
o Is it even a good idea to have more than one RAID group per volume?
Say I fill up a volume on a 14-disk RAID group (12d+1p+1hs), and then I add two more shelves (13d+1p) in another RAID group to expand this volume. (This gets rid of the RAID group size imbalance that will definitely result in poorer performance, which is the next subject discussed after the quote above.) Say I continue to add data to the volume, and as is likely, the last added data is probably the most often used. Will I have a "hot spot" in the second RAID group with the consistency points, since I'll be doing lots of reads and writes to the new data and very little to the old data?
o If the next consistency point is the one that is going to update RAID group #2, where does the data go that needs to be written to RAID group #1?
o Can the filer get into a state of hurryup!...wait...hurryup!....wait in the above example, or in the example of severely imbalanced RAID group sizes in the same volume? Or, do the consistency points get written out sooner than every 10 seconds even if half the NVRAM isn't filled, in order to "catch up" with the more active RAID group?
I'm not sure if these are stupid questions or so esoteric as to be practically meaningless. Nevertheless, I'm curious.
[0] Another Monty Python veiled reference. For some reason, I'm on a kick lately...
And Hitz spake, saying, 'First shalt thou take out the Holy Failed Disk. Then, shalt thou count to three, no more, no less. Three shalt be the number thou shalt count, and the number of the counting shall be three. Four shalt thou not count, nor either count thou two, excepting that thou then proceed to three. Five is right out. Once the number three, being the third number, be reached, then lobbest thou thy Holy Failed Disk of NetApp towards thy foe, who, being naughty in my sight, shall snuff it.'
Until next time...
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